1. Field of the Invention
The present invention relates to a rotating apparatus with a torque limiter function and is advantageously applicable to a compressor of a vapor-compression type refrigerator for a vehicle.
2. Description of the Related Art
A compressor of a vapor-compression type refrigerator for a vehicle is powered by the engine for moving the vehicle. The compressor or a pulley driven by a belt has a torque limiter function to prevent transmission of the torque when the transmission torque is above a predetermined value, in order to protect devices and components, such as a belt, provided on the drive source side, when a sliding part of the compressor is stuck due to burning, etc.
In a conventional, rotating apparatus with a torque limiter function, a breakable part whose diameter is smaller than those of the remaining portion is provided on a shaft of the compressor, so that the breakable part can be preferentially twisted and broken when the transmission torque is above a predetermined value in order to realize a torque limiter function.
In the above-mentioned solution in which power transmission is interrupted by breaking the easily breakable part provided on the shaft, the manufacturing cost can be reduced because the power transmission can be prevented by a simple structure, but the design and development thereof are made difficult for the following reasons.
That is, the strength of the breakable part must be such that it can be broken at a predetermined torque (hereinafter referred to as a breaking torque) T1. However, fatigue breakage (fatigue fracture) occurs at a torque smaller than the breaking torque, as is well known.
The maximum torque, i.e., an allowable torque T2, acting on the breakable part must be smaller than a value obtained by dividing the breaking torque T1 by a safety factor S.
If a ratio (=T1/T3) of the breaking torque T1 to the maximum value of torque (hereinafter referred to as a necessary transmission torque) T3 that must be transmitted, is smaller than the safety factor S, the necessary transmission torque T3 exceeds the allowable torque T2 and, hence, power cannot be transmitted.
In general, the safety factor S must be made larger as theoretical analysis of the stress is made more difficult due to a complicated state of stress that occurs in a structure. Therefore, if the stress that occurs in the breakable part can be precisely calculated and analyzed, the safety factor S can be reduced and the allowable torque T2 can be increased.
The above-mentioned breakable part is broken mainly by twisting, i.e., a shearing force (tangential stress). As the shearing force tends to be concentrated on a surface, it is difficult to precisely calculate and analyze the stress distribution.
In a structure similar to the above-mentioned rotating apparatus, because the dimension and the material, etc. of the breakable part must be determined by trial and error, there are difficulties in designing and developing the same and the actual breaking torque varies, thus leading to a fear of malfunction of the torque limiter such as an occurrence of breakage of the breakable part by twisting at a torque lower than the breaking torque.